Low-density lipoprotein (LDL) is a highly compartmentalized particle consisting of an outer monolayer of phospholipids, a cholesteryl ester core, and lipophilic antioxidants (e.g. vitamin E, -carotene, ubiquinol) distributed throughout the particle. Oxidative modification of LDL has been hypothesized to be a key event in the formation of an atherosclerotic lesion. The mechanism(s) of this stress remain elusive. LDL also contains a single apolipoprotein B-100 (apoB) interjected in the particle. ApoB contains a total of 25 cysteine residues: seven exist as free cysteines, which are localized in a hydrophobic region of LDL, and the remainder form disulfide linkages. Although the cysteine residues present in apoB were previously implicated in radical-scavenging antioxidant reactions in LDL, there exist to date no temporal data on apoB cysteine depletion and LDL oxidation. Previous studies implicate peroxynitrite as an oxidant involved in the process of atherosclerosis. Immunohistochemistry, nitrotyrosine, a product of peroxynitrite attack on proteins, can be seen in advanced lesions of atherosclerosis. In vivo studies have shown that native LDL treated with low levels of peroxynitrite is more susceptible to Cu2+-mediated oxidation as measured by a decreased lag time for the formation of conjugated dienes. We postulate that apoB thiols may act as an endogenous antioxidant defense against oxidative modification of LDL. In this study, we investigated the possible antioxidant role of apoB thiols during Cu2+/peroxynitrite-mediated oxidation of LDL and examined the effects of peroxynitrite oxidation of cysteine and tyrosine residues of apoB.